The invention relates to a method and to an apparatus for the machine soldering of workpieces, in particular circuit boards or substrates carrying circuit components, wherein the workpieces are moved by means of a forwarding system one after the other to the individual treatment stations, such as a fluxing station, a drying and/or conditioning station and finally to the actual soldering station.
The workpieces involved are primarily circuit boards or substrates for electronics and for microtechnology.
When soldering circuit boards and other electronic modules the cycle of movements is normally determined by the workpiece, i.e. the workpieces are continuously subjected to an individual movement within the workpiece flow system and are guided over stationary modules of automatic soldering units where, for example, wetting with flux (fluxing), pre-drying/pre-heating and soldering take place one after the other. In known automatic soldering machines the quality of soldering depends largely on the workpiece carriers, the tolerances between the workpiece carriers and the transport system and the tolerances between the workpieces and the carriers. These tolerances originate, for example, from the thermal expansion of the circuit boards within the mounting elements in the workpiece carriers.
It is also known that the material flow within the automatic soldering units must take place at a speed which is as high as possible in the interests of high throughput.
Accordingly, in the known methods, the individual modules and conveyor lines are dimensioned to ensure that the times required for example for pre-drying and for fusing of the melt, are available at the required speed of operation. If a particular construction cannot be made larger for space or time reasons (for example in the region of the solder bath or the solder wave) it is necessary, in known devices, to match the transport speed of the whole system to these determining factors which frequently leads to uneconomical operation.
In the known soldering system it is not possible, within a production series, to change the parameters as the workpiece passes through the system because such changes can in general only be effected by changing the speed of throughput. This means that in practice only one sort of workpiece can be dealt with by an automatic soldering plant at any one time. Change-over between different workpieces can involve relatively long downtimes and inefficiency particularly with small batch production. Furthermore, in known soldering systems extended arrangements and/or waiting stations are required, which also makes the transport system station dependent (in order to transfer the required energy to the workpiece (for example thermal energy for pre-drying and for soldering) while it is passing through a particular station). The flow of energy must therefore, in some cases, take place over correspondingly long transport paths in order to act on a workpiece with the required quantity of energy during its movement through a particular station. This energy is however continuously radiated even during empty running. The quantity of energy consumed in this way reduces the degree of efficiency and the waste of energy can hardly be justified.
Practice has shown that stoppage times (cycle times) of the workpieces must be expected both when manufacturing electronic modules (for example circuit boards) manually and also inside automatic mounting machines. Previous soldering methods function anti-cyclically to the preceding and subsequent working steps because the soldering process takes place while the workpieces are moving. In the known methods it is therefore always necessary, having regard to the soldering process, to complete the movement of workpieces, which on its own account could be completed rapidly, within accurately defined limits for example at certain speeds by using suitably adapted workpiece carriers. In order to ensure speed compensation between the speed of the transport system and the speed of the soldering process it is necessary to use additional buffer members with corresponding control complexity.